Aluminum filters and method of production



March 3, 1959 E. D. FISHER ALUMINUM FILTERS AND METHOD OF PRODUCTION Filed March 28, 1957 AL PORE AMMONIUM FIBERS MIX FORMER BICARBONATE OR 50-1004, 040% AMMONIUM BY BY WEIGHT TRl-CHLORIDE WE'GHT AL. POWDER DIE PRESS To DESIRED sHAPE so-loo% "GREEN" BY'WEIGHT 7000 To 20,000'RSJ.

SUBMERGE "GREEN" ELEMENTS MEL-r HEAT IN a a To 50C ALCL3- NA CL MoLT N FLUX 500 c To 600 c 1 19x To 4 HOURS C QUENCH IN HOT WATER TO REMOVE FLUX CLEAN BY SOAKING IN SOLUTION OF 10 PARTS HNO; 2 PARTS HF H20 2 MINUTES -REPEAT RINSE IN HOT WATER TO REMOVE SALT FOR 30 To 120 sEcoNos -REPEAT AIR DRY FIG. I

INVENTOR. ELWYN DEAN FISHER BY W 2 TORNEY United States Patent ALUMINUM FILTERS AND METHOD OF PRODUCTION Elwyn Dean Fisher, Cleveland, Ohio, assignor to Purolator Products, Inc., Rahway, N. 1., a corporation of Delaware" Application March 28, 1957, Serial No. 649,106

5 Claims. (Cl. 75-223) created in the green compacts. The sintering treatment of aluminum and other readily oxidized materials, however, is complicated by the film of oxide on the particles or fibers which interferes with metal to metal bond between the particles or fibers. This interference ordinarily results in unsatisfactory fragile elements that have no T commercial utility. It is necessary to provide special j' process features to eliminate the oxide film and to permit sintering to occur.

Principal features and objects of the present invention are the provision of a sintering treatment for aluminum elements which will achieve sufficient removal of the oxide film and permit the required effective sintering to occur.

Other objects and features of the invention are the provision of effectively sintered porous aluminum metal elements that are'strong, have satisfactory fiow rate and relatively low pressure drop as well as controllable pore size, that may be used for filters, heat exchangers, bearings and the like.

Further objects and features of the invention are the provision of practical methods of preparing commercially useful elements of sintered aluminum fiber or powder particles for use as filters, heat exchange or heat transfer elements, bearings or for other general uses.

In practicing the invention a mass of metallic aluminum particles such as fibers or powders or combinations of both with or without added pore or void formers such as ammonium bicarbonate, aluminum trichloride or the like, are given a desired green shape by pressure for example in a mold or die. In this pressing action, the oxide film on the particles is ruptured, bringing clean metal in adjacent particles into contact. Then the shaped- ,green elements are submerged in molten flux heated to the sintering temperature of the aluminum for a suffi- :cient length of time to remove oxide film and keep the inetal surfaces clean and free of exposure to oxygen and to permit sintering junction of the particles to occur while in the heated flux. Thereafter the sintered elements are i'emoved from the flux, washed to rid them of fiux and then cleaned in an acid bath and brightened in a brightening bath. After final washing the sintered, cleaned and brightened elements are dried and ready for use.

, Additionally, the invention contemplates the use of other readily oxidizable metal particles alone or in combination with aluminum for the production of sintered filter elements. In such practice the flux selected for use during sintering of the green" elements is selected to be compatible with the metal or metals used and effective in removing the oxide film and allowing effective sintering to occur.

2,876,097 Patented Mar. 3, 1959 Additional objects and features of the invention will become apparent from the following description and the accompanying drawing wherein:

Figure 1 represents a flow' diagram of a method of practicing the invention; and

Figure 2 illustrates in perspective an aluminum filter element prepared in accordance with said method.

Referring to the drawing, the reference numeral 10 denotes a porous sintered aluminum filter element prepared in accord with the process of this invention.

In a specific example of practicing the invention, aluminum metal fibers of the order of A" to long 200 mesh diameter and aluminum powder of the order of 200 mesh screen size preferably in equal parts by weight are mixed together thoroughly with the addition of up to 10% by weight of a void or pore former such as ammonium bicarbonate or ammonium trichloride.

The mixture is placed, for example, in a cylindrical die lubricated, for example, by lithium or calcium stearate and compressed under a pressure of from approximately 6,00020,000 p. s. i. to produce cylinders called green compacts. Other desired green compact shapes may be effected by such compression. The high pressure used is believed to rupture the oxide film and allow contact of clean metal in adjacent particles and also permit ready removal of the oxide by an appropriate flux in sintering operations. Sintering steps are now effected on the green compacts in the presence of an appropriate aluminum fiux.

A preferred flux found useful for sintering purposes is a sodium chloride-aluminum trichloride flux of the type described in Wagner U. S. Patent 2,723,929, dated November 15, 1955, and preferably consisting of a homogeneous mixture of from 70%90% by weight of aluminum trichloride and 30%-10% by weight of sodium or other alkali metal chloride. This flux is melted at approximately 120 C. becoming a clear liquid and is then heated to an aluminum sintering temperature of from 500 to 600 C.

The green compacts are deposited and submerged in the clear and heated molten flux bath over which an inert blanketing atmosphere as of argon is maintained and are maintained therein at the sintering temperature of from 500 to 600 C. for a period of approximately 1 to 4 hours during which sintering of the aluminum particles and their junction by sintering occurs. The flux described has been found effective. Other fiuxes and atmospheres are contemplatedas useful in practicing the invention.

Upon completion of the sintering step, the sintered compacts in the form of tubes are removed from the flux bath in the inert atmosphere, for example, argon and cooled to about 100 C. They then are quenched in hot water and allowed to soak therein to remove flux and, if present, the pore forming ammonium bicarbonate from the pores of the sintered tubes. This soaking is carried out long enough to leach the major portion of the flux and other materials from the pores.

Following this water leaching step, the tubes are cleaned, for example, by soaking in a solution of 10 parts of nitric acid HNO and 2 parts of hydrofluoric acid HF and water for about two minutes. This acid soaking is followed immediately by a hot water rinse and the acid soaking and rinsing steps are alternated until all salt has been removed from the sintered elements.

Thereafter, the acid soaked, water rinsed, clean elements are brightened in a brightening bath maintained at a temperature of from 50-110" C. into which they are dipped for 30-120 seconds at a time, followed immediately by a hot water rinse repeated alternately until the surfaces of the tubular elements are bright. The

preferred 70% Phosphoric acid 3% nitric acid 12% acetic acid 15% water Other appropriate brightener baths may be used.

The thinner the walls of the elements the faster the brightener bath-acts and consequently its temperature and the time of dipping therein respectively must be lower and shorter. Careful correlation to wall thickness is necessary.

After a final hot water rinse, the cleaned and brightened, sintered, tubular, aluminum filter elements are dried in air or in an oven at a slightly elevated temperature ranging from 40-50 C.

The dried products resulting are the desired tubular filter elements of this invention.

Although the invention has been described specifically with reference to aluminum elements, other readily oxidizable metals can be processed similarly by the use of appropriate fluxes that will rid them of their oxide films during sintering operations.

The porosity and other. qualities of these elements depend on a number of variables, namely:

(a) Proportion of powder and pore former (b) Size and type of powder particles Temperature of sintering steps (d) Duration of sintering process steps (e) Wall thickness of the elements Variations in any of these factors may be used as controls to vary porosity of the elements, rate of flow therethrough, strength and length of life as well as other qualities of the filter elements produced.

An alternative procedure for producing aluminum elements of desired shape is to prepare a molten bath of the hereindescribed flux, then cool this bath to viscous state and add to it particles of aluminum] powder or fibers or a mixture of these two providing a plastic mass,

then shaping the plastic mass to desired green form of the elements as by pressure or extrusion, and thereafter heating the "green formed elements, for example, in an inert argon atmosphere to a sintering temperature of from 500-600 C. Such procedure will permit production of sintered simple strip or block-like shapes. With tubular or convoluted shapes, however, provision must be made for support of the shaped elements during sintering to prevent disintegration. Such support may, for example, be provided by packing the green" forms with a refractory material during sintering.

Still another alternative procedure comprises mixing particles of aluminum powder alone or fibers alone or mixtures of these two with sodium chloride and hot pressing such mixture into desired shape at temperatures of from 385'-400' C. for a period of time sufficient to effect sintering union of the aluminum particles and/or fibers. The temperature and pressure in relation to time will effect the extent of sintering effected in the final product. I

While, therefore, preferred methods of pracu'cing the invention have been described, variations within the scope of the appended claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact details shown and described.

What is claimed is:

1. A process of preparing sintered aluminum elemenm comprising the steps of compressing a mixture of aluminum particles of small size and of an aluminum pore former compound, into desired shapes thereby forming "green" compacts, providing an aluminum flux consisting of an intimate homogeneous mixture of 70%-90% by weight of aluminum trichloride and %-10% by weight of alkali metal chloride, melting this flux and heating the molten flux to an aluminum sintering temperature of from 500-600 C., submerging the green compacts in this molten and heated flux for a period of from 1-4 hours, thereafter removing said compacts from said bath as sintered aluminum elements and then "cleaning and brightening said elements.

2. A process of preparing sintered aluminum elements comprising the steps of compressing aluminum particles into desired shapes forming green compacts, providing a molten aluminum flux bath comprising aluminum chloride and alkali metal chloride, heating the bath to an aluminum sintering temperature of from 500-600 C., submerging the "green compacts in said molten hated flux bath for a period of from 1 to 4 hours to effect sintering union of the aluminum particles in the green" compacts to form sintered aluminum elements, thereafter removing the elements from the bath, leaching the same with water to rid them of retained flux, then acid cleaning the leached elements, and, thereafter brightening the acid cleaned elements in a brightener bath.

3. The process of claim 2 wherein said brightener bath comprises a solution containing approximately phosphoric acid, 3% nitric acid, 12% acetic acid and 15% water.

4. The process of claim 2 wherein the acid cleaning is effected in a bath which consists of approximately 10 parts of nitric acid HNO; and '2 parts of hydrofluoric acid HF mixed with water in the proportion of'80% water and 20% nitric acid hydrofluoric acid mix.

5. A process of preparing sintered aluminum elements comprising the steps of providing aluminum fibers and aluminum powder, adding a pore former to form a mixture, compressing the mixture at a pressure ranging from 6,000 to 20,000 p. s. i. to form green" compacts, providing a molten aluminum flux bath, elevating the temperature of the molten flux bath to from 500'-600' C., submerging the green" compacts in this molten flux bath at said temperature for a period of from 1 to 4 hours to effect sintering junction of the aluminum fibers and powder, then removing the compacts from the molten flux bath, then leaching the compacts with water to rid them of retained flux and pore former, thereafter eflecting acid cleaning of the leached compacts in an acid cleaning bath comprising nitric acid, hydrofluoric acid and water and thereafter brightening the cleaned compacts in a brightener bath comprising a mixture of phosphoric acid, nitric acid, acetic acid and water.

References Cited in the file of this patent UNITED STATES PATENTS 2,319,240 Larsen May 18, 1943 2,470,034 Hensel May 10, 1949 1 2,491,866 Kurtz Dec. 20, 1949 2,546,320 Rostron Mar. 27, 1951 2,723,929 Wagner Nov. 15, 1955 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. I, 1949, pp. 574, 576. 

1. A PROCESS OF PREPARING SINTERED ALUMINUM ELEMENTS COMPRISING THE STEPS OF COMPRESSING A MIXTURE OF ALUMINUM PARTICLES OF SMALL SIZE AND OF AN ALUMINUM PORE FORMER COMPOUND, INTO DESIRED SHAPES THEREBY FORMING "GREEN" COMPACTS PROVIDING AN ALUMINUM FLUX CONSISTING OF AN INTIMATE HOMOGENEOUS MIXTURE OF 70%-90% BY WEIGHT OF ALUMINUM TRICHLORIDE AND 30%-10% BY WEIGHT OF ALKALI METAL CHLORIDE, MELTING THIS FLUX AND HEATING THE MOLTEN FLUX TO AN ALUMINUM SINTERING TEMPERATURE OF FROM 500*-600*C., SUBMERGING THE "GREEN" COMPACTS IN THIS MOLTEN AND HEATED FLUX FOR A PERIOD OF FROM 